Adding on

The function on defined by

(*) `on` f = \x y -> f x * f y

is convenient when using functions like sortBy: sortBy (compare on fst), for example. It also makes the code more readable.

Furthermore I consider on to be above the Fairbairn threshold, since

• we get rid of two lambdas,

• we get rid of the duplication of p,

• on has some nice algebraic properties (documented in the

  patch)

• and, most importantly, it is easier at a glance to understand (*) on p than to understand \x y -> p x * p y (assuming one knows about on).

The main question seems to be which module to put the function in. Based on previous discussion on the libraries list I suggest creating a new module Control.Function containing on plus (initially) some other combinators (working solely on and with functions) from the Prelude.

Deadline for discussion: 2006-11-09.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown